The present invention relates to integrated-circuit technology and, more particularly, to a method for post-manufacturing custom-modification of an integrated circuit. A major objective of the present invention is to provide a simple, economical, and reliable method of linking metal conductors of an integrated circuit.
Advances in integrated-circuit manufacturing technology have allowed ever increasing functionality to be implemented on a single device. A typical integrated circuit includes: a silicon substrate with regions doped to control their conductivity type, field and gate oxides for electrically isolating the substrate from overlaying conductors, one or more polysilicon layers for forming contacts and local interconnects, a submetal dielectric layer, a metal interconnect structure, and a passivation layer.
The metal interconnect structure can have one or more metal layers. Where there are plural metal layers, the layers are separated by intermetal dielectric, usually at least as thick as the underlying metal. In addition, the submetal and intermetal dielectrics are often planarized so that the next metal layer can be formed on a flat surface. The passivation layer tends to be thinner, since it is not required to isolate conductors, and not planarized, since it is not used as a base for subsequently formed features.
As integrated circuit designs have become more complex, it has become increasingly difficult to ensure that a design properly implements all its intended functions. Accordingly, prototypes must be built and tested before a commitment is made to a large volume run. However, small-volume prototype runs can be quite expensive and time consuming. If a design defect is found in a prototype, it is desirable to verify the new design prior to starting over to make a new prototype.
One increasingly popular prototyping approach uses electrically (e.g., "field", or "user") programmable devices. The functions performed by such devices are determined after manufacture by electrically programming the device. If defects in the programming of a first device are discovered, a second device can be correctly programmed and substituted for the first device. Even more convenient are electrically reprogrammable devices in which a defective design program can simply be overwritten by a corrected program. However, the variations that can be implemented by programming are limited by the circuit as designed into the programmable device. If the circuitry of the programmable device is defectively designed, the defect cannot in general be corrected by reprogramming the device or by substituting a nominally identical device with different programming. Thus, even with programmable and reprogrammable devices, the problem of correcting defectively designed hard circuitry remains.
In an "antifuse" approach to electrically programming a circuit, a large voltage differential is applied across two adjacent conductors (on the same or different layers) so that the intervening dielectric breaks down, thus creating a link. A problem with the antifuse approach is that the resulting link has relatively high impedance, so large currents, e.g., driver currents, cannot be handled. Furthermore, the antifuse approach is limited to creating links, whereas it is desirable to be able to create reliable opens along existing conductors as well.
It is also possible to physically modify an integrated circuit to create an open. For example, a laser can be used to cut a conductor to create an open. However, the laser energy disrupts the overlaying dielectric, impairing the predictability of the modification as well as the long-term reliability performance of the circuit.
Focused ion beam systems have been used to make connections as well as break them. A focused, rasterized beam of high-energy ions, e.g., gallium ions, can be used to sputter and remove dielectric over metal lines. If a break is desired, the beam can be used to precisely cut through the metal. If a new connection is desired between thus exposed conductors, the metal itself is left undisturbed; metal-bearing, e.g., tungsten carbonyl, gas is admitted into the vacuum chamber. The ion beam is scanned from one metal electrical node to the other. The ion beam locally decomposes the metal-bearing gas adsorbed onto the surface, leaving a conductive trace between the metal electrical nodes. This technique is very flexible, permitting connections even between nodes that are on the same or different metal interconnect levels and disposed far apart on the integrated circuit.
The main advantage of the focused ion beam approach is that it allows flexible modification of an integrated circuit, both before and after integrated circuit manufacture is completed. The focused ion beam approach is costly in that the equipment is expensive and requires a high degree of skill on the part of the operator. If circuit breaks are required in isolated lines, the laser cutting approach is most cost effective. However, as with laser cutting, the focused ion beam damages intermetal and passivation dielectric, the effects of which can be difficult to predict and control. Accordingly, devices so modified are best limited to design verification purposes; newly designed and manufactured devices are still required for end uses.
What is needed is an improved method of modifying integrated circuits. It should provide for making connections as well as breaking them. Yet, the method should be more economical and less destructive than the focused ion be a m method.